1. Field of the Invention
The present invention relates to a web transporting mechanism of a printing apparatus for forming an image on a web continuously transported. More particularly, relates to the web transporting mechanism of the printing apparatus having a buffer mechanism for absorbing a fluctuation of a web length between a transporting section and a fixing section.
2. Description of the Related Art
In a printing apparatus for forming an image on a continuous web, the web is transported in such a manner that a pin member of a tractor mechanism mounted on the printing apparatus is engaged in a sprocket hole of the web of paper and the web is transported when the tractor mechanism is driven. In the case where the web having the sprocket holes is used, it is necessary to cut off both end portions of the web, in which the sprocket holes are formed, after the completion of printing. Therefore, a printing apparatus has been put into practical use in which a web having no sprocket holes is used and the web is transported by a transporting roller mechanism instead of the tractor mechanism so that the work of cutting both end portions of the web can be avoided.
The above printing apparatus includes: a transporting section for recording and forming an image on a web; and a fixing section for fixing the image onto the web by heating and pressurizing toner which has been transported onto the web in the transporting section, wherein the web is discharged from the fixing section. However, in this structure, a transporting member (Hereinafter, referred to as transporting means,) for transporting the web in the transporting section and the transporting means for transporting the web in the fixing section are independently driven. Accordingly, it is difficult to perfectly make the web transporting speed in the transporting section coincide with the web transporting speed in the fixing section.
When the web transporting speed in the transporting section is higher than that in the fixing section, looseness of the web is caused between the transporting section and the fixing portion. When the web is continuously transported by the transporting section and the fixing section, the degree of looseness is increased. On the contrary, when the web transporting speed in the fixing section is higher than that in the transporting section, a sufficiently long web cannot be supplied from the transporting section to the fixing section. In order to solve the above problems, a buffer mechanism absorbs a fluctuation in the web length between the transporting section and the fixing section caused by a difference in the web transporting speed generated between the transporting section and the fixing section.
An example of the buffer mechanism of a conventional printing apparatus is shown in FIG. 3 and explained as follows.
There is provided a movable buffer 4, the position of which is changed according to the length (referred to as a web length L hereinafter) of the web 3 between the transporting section 1 and the fixing section 2. The movable buffer 4 is pushed to the web 3 by the twist coil spring 5, and a position of the movable buffer 4 is changed according to the web length L. The position of the movable buffer 4 is detected by the sensor 6. According to the detection signal, the web transporting speed in the fixing section 2 is controlled. Explanations will be made into a case in which the web transporting speed of the fixing section 2 is controlled by the rotating speed of the heat roller 7 and that of the pressure roller 8. In the case where it is detected that a position of the movable buffer 4 is higher than the target position (the wab length L is longer than the target value), the rotating speed of the heat roller 7 is increased so that the web length L can be shortened. On the contrary, in the case where it is detected that a position of the movable buffer 4 is lower than the target position (the wab length L is shorter than the target value), the rotating speed of the heat roller 7 is decreased so that the web length L can be extended. In this case, the explanations are made into the case in which the web transporting speed in the fixing section 2 is changed. However, it should be noted that the present invention is not limited to the above specific system. When a difference between the rotating speed of the web transporting rollers 9a, 9b, which determines the web transporting speed of the transporting section 1, and the rotating speed of the heat roller 7, which determines the web transporting speed in the fixing section 2, is made to be variable by the web transporting speed control circuit 10 according to the detection signal of the sensor 6 to detect the position of the movable buffer 4, the same effect can be provided. (See JP-A-9-146316.)